How to animate Texture change in Blender Internal
You may shift textures animating the Offset value of the gradient texture, which works as a mask for transition between two textures.


Prepare two different textures.


Add a new material to your surface and unwrap it. Give it two image textures (set their type to Image or Movie and mapping coordinates to UV). Now add one more texture. Set its type to Blend and place it between previously added ones. In Influence panel uncheck the Color box and check Alpha. Check the RGB to Intensity and Stencil boxes. 


Now the gradient texture acts like a mask. In Colors panel check the Ramp box and use sliders to define the smoothness/sharpness of the transition between textures.


In Mapping panel change the Offset value of the X window so you can see only one texture. Hovering your cursor over that window press I to insert a keyframe. Go to another frame, change the Offset value and press I again.